1. Field of the Invention
The present invention relates generally to an injection mold and a method of producing a molded resin article, and more particularly to an improvement of an injection mold suitably used for producing a molded resin article including a cylindrical portion or a deep hole portion by injection molding, and a method of advantageously producing such a molded resin article by injection molding.
2. Description of Related Art
As a method of producing a molded resin article, an injection molding method is known and used for producing various kinds of molded resin article. In production of the molded resin article by the injection molding method, there is used a so-called injection mold having a cavity for forming the molded resin article, a runner for introducing a molten resin injected from an injection machine into the cavity, and a gate for communication between the runner and the cavity. The quality of the molded resin article obtained as an end product is highly dependent on arrangement of the gate formed in the injection mold.
Namely, in the production of the molded resin article including a cylindrical portion or a deep hole portion, in a case where the gate is formed at a circumferential position of a cylindrical space of the cavity, which space gives a side wall of the cylindrical portion or the hole portion of the intended molded resin article, for example, the molten resin flowing through the gate into the cavity is circulated through the cylindrical space of the cavity in its opposite circumferential directions, whereby fronts of flows of the molten resin come together within the cavity. As a result, a weld line is likely to be formed in the side wall of the cylindrical portion or the hole portion of the obtained molded resin article, at the circumferential position where the fronts of the flows of the molten resin come together, whereby the molded resin article has a poor appearance.
Under the above-described circumstances, JP-A-59(1984)-143622 discloses an injection mold which permits production of a cylindrical molded resin article by controlling the problem of the poor appearance of the molded resin article due to formation of the weld line or the like, for example. In this injection mold, the runner has: a first communication hole which extends toward the cavity and one of opposite ends of which communicates with the gate; and a second communication hole which communicates with (which is connected to) the other end of the first communication hole remote from the gate, and which extends in a direction perpendicular to the direction of extension of the first communication hole. Further, a pin member is disposed within the first communication hole, so as to be spaced apart from an inner circumferential surface of the first communication hole and to extend coaxially with the first communication hole, and such that a distal end portion of the pin member extends through the gate. Thus, a ring runner portion is defined by a cylindrical space between the inner circumferential surface of the first communication hole and an outer circumferential surface of the pin member, which surfaces are opposed to each other, and the gate takes the form of an annular ring gate. In the injection mold described above, the molten resin is introduced through the ring runner portion and through the ring gate into the cylindrical cavity, and flows into the cavity in its axial direction. Thus, this injection mold is intended to prevent fronts of flows of the molten resin from coming together within the cavity, and accordingly prevent formation of the weld line, which would otherwise be caused where the fronts of the flows of the molten resin come together.
However, the inventors of the present invention made various studies on the conventional injection mold constructed as described above, and found that the formation of the weld line in the molded resin article obtained as the end product cannot be sufficiently prevented by using the conventional injection mold.
Namely, in the conventional injection mold, the runner has: the ring runner portion which is defined by the space between the inner circumferential surface of the first communication hole and the outer circumferential surface of the pin member, and which extends toward the ring gate; and a straight runner portion defined by the second communication hole extending in the direction perpendicular to the direction of extension of the ring runner portion. Further, one of opposite ends of the straight runner portion, which is remote from the ring runner portion, communicates with a sprue. Therefore, in the conventional injection mold described above, when the molten resin, which has been injected from an injection machine and flowed through the sprue and the straight runner portion, flows into the ring runner portion at a circumferential position at which the straight runner portion and the ring runner portion communicate with each other, a pressing force generated by a pressure of the injected molten resin is applied to the pin member disposed within the ring runner portion, whereby the pin member is pressed away from the above-indicated circumferential position. At this time, in a case where the molten resin includes a glass fiber or the like and has a low degree of fluidity, the molten resin is injected from the injection machine at a particularly high pressure, so that the pin member is pressed by a high pressure of the injected molten resin, and an axis of the pin member within the ring runner portion may be displaced due to flexural deformation of the pin member, for example. In this case, an amount of flow of the molten resin through the ring runner portion in its axial direction (direction of its extension) toward the ring gate may be uneven in a circumferential direction of the ring runner portion, causing fronts of flows of the molten resin to come together within the cavity, and leading to formation of the weld line in the molded resin article obtained as the end product.